Add Gemini CLI PR review

[RissyRan]

Description

Add Gemini CLI PR review feature to MaxDiffusion repo, and also add instructions into developer guide.

Source from MaxText.

Tests

Tested in this PR.

Checklist

Before submitting this PR, please make sure (put X in square brackets):

• I have performed a self-review of my code.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have run end-to-end tests tests and provided workload links above if applicable.
• I have made or will make corresponding changes to the doc if needed.

[github-actions]

e2e testgrid: https://8bcf50593faf4ea38060e236169827e5-dot-us-central1.composer.googleusercontent.com/dags/maxdiffusion_tpu_e2e/grid

[github-actions]

🤖 Hi @RissyRan, I've received your request, and I'm working on it now! You can track my progress in the logs for more details.

[github-actions]

## 📋 Review Summary

This pull request introduces a GitHub Actions workflow for automated code reviews using the Gemini CLI, which is a great addition to improve code quality and automate the review process. The workflow is well-structured and includes important security considerations.

🔍 General Feedback

• The overall implementation of the GeminiReview.yml workflow is robust and follows best practices for GitHub Actions.
• The .gitignore update is correct and necessary for the new workflow.
• I've pointed out a minor issue regarding an undefined environment variable in the workflow configuration. Addressing this will improve the clarity and maintainability of the workflow.